"When somebody says 'ambush,' you see a character in a movie and you expect a guy to trip over a line or somebody pop up from a garbage can or somebody has the high ground and shoots on them," said Steve Weiss, research director for the Officer Down Memorial Page, whose website tracks slain U.S. law officers.

The Colorado, Texas, California and West Virginia deaths "are kind of like that movie-style ambush," Weiss said.

It's what many officers fear most, said CNN contributor Tom Fuentes, a former FBI assistant director.

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"Rookie officers are taught generally you're not concerned about the bullet with your name on it, but about those addressed 'To Whom It May Concern,'" Fuentes said, referring to random ambush shootings against police.

While an ambush often refers to an assailant lying in wait, the FBI statistics include "unprovoked attacks" without hiding, which one analyst likened to the circumstances in the Texas and California slayings.

The FBI counted 15 officer deaths by ambush in each year of 2011, 2010 and 2009, and said that ambush situations were the biggest category of circumstance behind 543 officers feloniously killed between 2002 to 2011: 23.2%.

The next highest category was an arrest situation, at 22.5%, according to FBI statistics.

Weiss' group doesn't specifically count ambush deaths of officers, but a search of its records with the word "ambush" shows that 14 law officers died in such a manner in the past 12 months -- a figure almost matching the FBI's recent data trends.

The group says that as of Friday, the total number of law enforcement fatalities is up 29% over the same period last year, and the number of gunfire-related fatalities of law officers is up 36%, Weiss said. There have been 31 officer fatalities in 2013, and 15 of them are gunfire deaths, Weiss said.

The killings in the four states have contributed to that increase, Weiss said. His group, however, doesn't include the two prosecutors' deaths in its counting.

Colorado prison chief Tom Clements was fatally shot at his home this month after he answered the door bell.

The Kaufman County, Texas, district attorney and his wife were shot to death in their home last month; his assistant D.A. was fatally gunned down in the parking lot of the county courthouse after a very short confrontation in January.

And rogue ex-Los Angeles Police Department officer Christopher Dorner threatened to kill police randomly and in fact shot five officers, killing two earlier this year. A Riverside police officer was fatally shot after Dorner drove up to his vehicle at a stoplight and fired. A San Bernardino County sheriff's deputy was shot by Dorner while responding to a report of a vehicle stolen by someone matching Dorner's description.

The sheriff of Mingo County, West Virginia, was killed this week after a gunman walked up to the sheriff as he sat in his vehicle in a parking lot near the county courthouse. The gunman shot the sheriff in the head and then returned to his vehicle and drove away.

The FBI defines ambush killings into two types. One is by "entrapment and premeditation," which is a scenario "where the officer was lured into danger as the result of conscious consideration and planning. These attacks are generally accomplished from cover or hiding; however, they can occur without cover or hiding," the agency said.

There are also ambushes of "unprovoked attacks," which are "generally accomplished without hiding; however, they can occur with or without cover,' the FBI said.

Weiss, however, questioned the FBI definition. From the viewpoint of police, what's considered a routine traffic stop can turn deadly when a motorist suddenly pulls a gun and shoots the officer.

"Unless you're responding to a scene where someone is shooting, pretty much anything is an ambush," Weiss said. "There's countless number of times where an officer stops a car for a traffic violation and then they are shot."

But Weiss acknowledged the ambush circumstances in California, Colorado and Texas, and noted how the Colorado killing evoked America's frontier era of the 1800s when local sheriffs were vulnerable to home attacks.

"When the public thinks of ambush, it's when someone calls 911 and there's somebody waiting on a rooftop. That's unusual," Weiss said. "They type of incidents in Texas and with the Colorado prison director is even more unusual.

"When somebody shows up at someone's home, those are really unusual. In these days, you very rarely see that," Weiss said. "Our town is a little more civilized than back in the 1800s."

"Any number above the number zero is high," Weiss said. "That is a high number. To make that decision ever is hard to fathom, but to plan it out in a premeditated matter is troubling."

But the overall yearly number of killed officers nationwide is much lower than the 1970s -- an era when violence on police was remarkably high, according to Weiss and Fuentes.

There were 120 officers killed in 2012, 176 each in 2011 and 2010, and 140 in 2009, Weiss said.

Those figures compare with 249 in 1971, 232 in 1972, 273 in 1973, and 279 in 1974, according to the Officer Down Memorial Page website.

Fuentes recalled the 1970s as an era where public respect toward police was low.

"With the Dirty Harry and Charles Bronson and 'Death Wish' movies, it was fair game to shoot cops on the street and there was generally a lack of respect for uniformed officers," Fuentes said. "But over the years, the respect has returned and been earned.